This invention relates generally to gas production compositions and more particularly to double base propellants not having an energetic plasticizer with secondary nitroxy groups.
The burning rate equation for double base propellants is given as r=KP.sup.n or log r=n log P+log K where r is the burning rate, P is the combustion chamber pressure, K is a constant for each propellant composition, and n is a constant for non-modified propellants but is a variable function in plateau propellants varying from very high positive values through zero to low negative values. Thus, a plot of log r against log P would give a straight line with a slope of n for a non-modified propellant, but a "plateau" shaped line for plateau propellants.
The plateau logarithmic relationship between the burning rate and the chamber pressure is greatly preferred over the linear logarithmic one. Such a relationship gives better ballistic and combustion stability, less dependence on initial temperature, and lower peak pressures in the combustion chamber.
In order to obtain the plateau relationship, additives referred to as ballistic modifiers are included in the propellant composition. These additives accelerate the burning rate at low pressures but have a decreasing catalytic effect as the chamber pressure increases up to a certain pressure. Then the decreasing catalytic effect diminishes and the rate-pressure relationship slowly approaches the rate-pressure relationship for the propellant without the modifier. Sometimes the rate-pressure function for the plateau propellant actually remains below that of the unmodified propellant.
Excellent plateau burning has been obtained for double base propellants plasticized by nitrate esters having secondary nitroxy groups, e.g., nitroglycerin. On account of the greater strength of the bond between a primary carbon and a nitroxy group, only moderate success has been obtained in formulating plateau double base propellants which are plasticized by an ester with only primary nitroxy groups. This is becoming an extremely serious problem to the propellant art because of increasing recognition of the superiority of the latter plasticizers and particularly of the energetic plasticizeer consisting of metriol trinitrate (MTN) and triethylene glycol dinitrate (TEGDN). Propellants made with the MTN-TEGDN plasticizer are vastly superior in health, stability, and storage considerations than propellants utilizing nitroglycerin as a plasticizer. For example such propellants have been stored for more than one year at 175.degree. F. without any appreciable loss in performance. This represents a storage life of at least three to four times that of nitroglycerin propellants.
The limited amount of plateau burning for this latter class of double base propellants has been achieved by the addition of large amounts, i.e., amounts in excess of two weight percent of certain lead and other metallic salts. Lead salicylate, lead .beta.-resorcylate, lead ethyl hexoate lead stannate, and lead acetylsalicylate have given the best results. Since these ballistic modifiers give results barely sufficient, the other ten or so known ballistic modifiers are not used in scaled up propellant systems unless mixed with the above lead salts.
The use of lead salts in any amount is objectionable on account of the health hazards caused by lead oxides in the exhaust gas and because of the problem of contamination of the worker's skin and clothing. A disadvantage of any metallic salt in the quantities now being used in the propellants contemplated by this invention is the increase in visibility of the exhaust due to the increase in the solids content of the exhaust gas.